In relatively inexpensive hand operated valves, typical valve construction requires positioning a valve stem packing around the stem. This packing surrounds the stem of the valve to prevent leakage along the stem. The packing is normally held in place within a bonnet assembly. The packing is axially loaded with a jam nut and bushing cooperative with the bonnet, this axially loading being necessary to radially expand the seal material and achieve sealing. Several undesirable aspects of this axial loading should be observed. First of all, the axial loading on the seal material forces the seal material to cold flow and it is extruded along the stem. If the pressure is high enough, the axial loading can force sufficient portions of the sealed material through the narrow encircling gap around the stem in the surrounding bonnet. This factor alone can limit the life of a seal.
Separate from this, the loading which is placed on a tubular sleeve used for isolation purposes forces the sleeve into a slightly distorted form. The form is described generally as bulging. Because the sleeve has significant length compared to the radial thickness of the sleeve, the bulges have the form of a wavy pattern, this pattern prevailing fully around the sleeve, thereby forming on a typical sleeve uneven wear surfaces in contact with the stem on the interior and a correspondingly offset encircling areas in contact with the surrounding bonnet. The wear is highly localized in the areas in contact with the stem. The undulations observed in the axially loaded sleeve are damaging to it, and create a localized surface damage amd wear. Leakage more often results with wear.
The later problem can be reduced by placing more load on the seal, and thereby further swelling the sleeve. This broadens the areas of contact on both sealing surfaces; it also increases the rate at which the sleeve material is extruded into the encircling gap around the stem.
Another problem arises in that changes in service cannot be accomodated so readily. For instance, one change in service results from a change in pressure requirements. Should the pressure requirements go up, the seal may not hold against the increased pressure. Rather, the increase in pressure can be resisted by increasing the loading on the seal. However, as the loading is increased, the life of the seal declines more rapidly. Softer seal material can be used and is quite acceptable for a short period of time, or a few operations of the valve. Long term use cannot be sustained with a soft seal material.
The seal of this disclosure is a marked improvement. It is able to overcome many of these limitations. In particular, it achieves sealing with a marked reduction in force. This reduces the bushing torque required. Moreover, the torque required on operation of the handle connected to the stem is reduced; this reduction decreases polishing or work hardening of the parts at their contact area.
With the foregoing in view, the present disclosure sets forth a seal mechanism which comprises an elongate hollow tubular sleeve formed of a flexible polymer material having slick faces, and is sliced into a plurality of thin parallel washers joined at the inner edge. The slices or cuts do not pass all the way through; they lack about 0.005 inches or slightly more from passing through the tubular sleeve, and the cuts enable the sleeve to act as a loose stack of washers or rings, each individually expanding. The several portions isolated by the cuts are able to deflect outwardly with a slight bow or bulge between cuts. At the cuts, the inside face adjacent to the stem bulges to form external and internal sealing edges.
In this manner, the device perfects a stem seal with reduced axially loading, thereby permitting reduced handle torque installation of a single stack or cartridge rather than several loose pieces reduces assembly and maintenance time and minimizes damage to seal parts.